NOBODY DOESN'T LIKE Sara Lee" and "The Impossible Dream" have in common that they were composed by the same man - Mitch Leigh, an unabashed extrovert from the streets of Brooklyn who also studied music at Yale under Paul Hindemith.

He is now 49, and one senses that Leigh's outsider beginnings made him cautious about the impossible dreams he found inside himself. Having composed the score for an overwhelmingly successful musical, "Man of La Mancha," which surely has given him financial independence over the past dozen years, Leigh has just divested himself of an even richer gold mine, Music Makers, Inc.

Now, with a full-scale "La Mancha" revival under way, with Richard Kiley playing his original Don Quixote for the first time in Washington starting Friday at the National, Leigh has two major works ahead - a musical this season to a book by N. Richard Nash and a full-scale opera almost completed.

Music Makers, Inc., is a ubiquitos but little-known influence on 217 million listeners. In the past 20 years, it has been pouring TV and radio commercials into not merely the 50 states but other parts of the globe as well. Leigh created this corporation when he glimpsed the fortunes advertisers were willing to spend in selling their wares.

The little jingles and catchy tunes were almost child's play to Leigh, who was responsible for hundreds of them. His favorites?

"I guess I'm proudest of the Benson and Hedges theme, which now stands on its own with subsequent lyrics, "The Disadvantages of You.' Have you heard Sinatra sing it? Then there's 'The Swinger,' which we did for Polaroid, the one with the voice over and face by Laurence Oliver. And I suppose the American Airlines theme has been, should we say, the most extensively heard."

Leigh is amused that we're interested, for he's found that people think it's, well, inappropriate that the fellow who has done so much to put factory cakes into family refrigerators is also the man who composed "The Impossible Dream," which began inspiring people shortly after "Man of La Mancha" opened and still does.

"In the early days," he muses, "non-musicians composed the commercials.It was a matter of a tune and very simple, rudimentary harmonies and really no arrangements.

"After Yale and Hindemith I was into jazz, usually playing the bassoon. I had a daily radio show and certainly knew my music. I could see how unsophisticated the commercials were and began to contribute more aware arrangements, using combinations of instruments previously untried. They caught on and Music Makers was the result.

"It became a very complex business, keeping track, for instance, of how often a jingle was used. ASCAP (the American Society of Composers and Publishers) isn't the only outfit that pays people to listen to every station. The staff just burgeoned, for records had to be kept for our own protection. No, I truly don't know how many commercials we've made over the decades, but the master tapes do exist in a barn in New Jersey."

Leigh smiles because he now is out of Music Makers - "I've got enough," he says succintly - and very much into the future, a future he feels he was trained to explore.

Leigh lives on Central Park South and city distractions don't bother him. "I'm told some people in the building complain about piano and practicing singers, but I don't hear them and they generally don't hear me.

"Of course I wanted to follow up 'La Mancha' with another success. My hopeful 'Cry for Us' was a flop. Before 'La Mancha' I'd had two musical stage failures. I did incidental music for an all-star revival of 'Too True To Be Good,' which didn't last long. If people heard the music, no one mentioned it.

"But it was produced and directed by Albert Marre and that led to 'La Mancha.'

"You know, 'Man of La Mancha' came from a TV show of 1959. 'I, Don Quixote.' Lee Cobb had the little part of the Sancho Panza was Eli Wallach.

"Well, its writer, Dale Wasserman, decided to rewrite it as a stage musical. Alby Marre became interested and brought in W. H. Auden for the lyrics and me for the music. Auden didn't work out and Alby then tapped Joe Darion for the lyrics.

"All that did work out, but getting the show on was something else. The only place in 1965 that would take a chance on us was Alebrt Selden's - later Michael Price's - then just re-opened Goodspeed Opera, built in 1876 and seating only 360. Kiley, who's worked with Marre in 'Kismet,' ageed to take the lead.

"Everyone loved the show, but knew it could never succeed in New York. John Chapman, the News critic, happened on it and wrote exactly that. Hal James, who was in the insurance business, came. He raised money in small peices, for nobody who'd backed shows before was interested. We all invested in it, everything we had. The only New York theatre we could get was one nobody wanted, the ex-ANTA Playhouse down on Washington Square.

"So, it was the impossible team. It did catch on all over the earth since.

"'Sarava', with a South American setting, is the play I'm doing with Nash, lyrics and musics. It will have a tryout in February at the University of Tennessee, Knoxville.

"The opera is from William Alfred's 'Hogan Goat.' There's a long story in back of this one for I already composed it once. Now we call it 'Matt' and I'm very much at home in it, about Irish politicians in Brooklyn in the 1980s.

"Now, Brooklyn, I know, and Irish politicians I know, and it's going to be a real opera."